Synthetic resin emulsion, easily water-swellable pressure-sensitive adhesive compositions containing the same and process for production of the emulsion

ABSTRACT

There is provided a synthetic resin emulsion comprising polymer particles having a core/shell structure, wherein the shell comprises a copolymer of an unsaturated carboxylic acid and a hydrophilic comonomer, the core comprises a copolymer of a monomer mixture comprising a radically polymerizable main monomer and a radically polymerizable functional monomer, and the monomers constituting the monomer mixture are selected so that the glass transition point (Tg) of the copolymer produced by polymerization is −20° C. or below, and the synthetic resin emulsion has been produced by adding the monomer mixture for core formation and a pH adjustor to an aqueous copolymer solution, which has not been neutralized, produced by polymerizing the unsaturated carboxylic acid and the hydrophilic comonomer in an aqueous medium, and allowing a polymerization reaction to proceed. The use of the synthetic resin emulsion can provide a pressure-sensitive adhesive which has excellent adhesive properties such as adhesive strength, cohesive force and tackiness and can be easily swollen with water without any treatment with an alkali.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a synthetic resin emulsion for aneasily water-swellable pressure-sensitive adhesive, an easilywater-swellable pressure-sensitive adhesive composition comprising thesame, and a process for producing the synthetic resin emulsion. Theeasily water-swellable pressure-sensitive adhesive composition issuitable for applications such as labels, tapes, building materials,packaging materials, and electronic materials.

2. Background Art

Elastic bodies such as natural rubbers and synthetic rubbers or acrylicresins have generally hitherto been used as components ofpressure-sensitive adhesives. In the formation of pressure-sensitiveadhesive labels, sheets, and tapes, these components are applied ontothe surface of a support. Such pressure-sensitive adhesive labels,sheets, and tapes can be easily bonded to the surface of an object atroom temperature by pressure around a finger pressure and thus havebecome extensively used in various applications.

For example, these pressure-sensitive adhesive labels, tapes and thelike are applied to adherends such as bottles, cans, or plastics. Afteruse, these adherends are often corrected and recycled. Thepressure-sensitive adhesive labels and the like applied onto theadherends, however, may become an obstacle to recycling of the adherendsin a recycling process.

In some applications, after the pressure-sensitive adhesive label, tapeor the like is temporarily used, the label, tape or the like is desiredto be removed at once from the adherend. For example, in polishing ofthe backside of a silicon wafer as an electronic material, apressure-sensitive adhesive sheet is used for supporting the wafer andprotecting a surface pattern. This pressure-sensitive adhesive sheet isseparated from the wafer after the polishing of the wafer. In this case,that the pressure-sensitive adhesive does not remain unremoved on thesurface pattern is required.

Pressure-sensitive adhesive sheet are also used in sanitary goods. Inthis application, easy treatment of filth by water washing is requiredof the pressure-sensitive adhesive sheet.

In recent years, various alkali-swellable pressure-sensitive adhesivesin which the pressure-sensitive adhesive can be dispersed or dissolvedby alkaline water treatment in the recycling process have been put topractical use. The alkaline water treatment necessary for this treatmentof the pressure-sensitive adhesive, however, possibly has an adverseeffect on environment. Further, in pressure-sensitive adhesive sheetsfor electronic materials and sanitary goods, such alkali treatment perse cannot be carried out without difficulties.

For this reason, the development of a pressure-sensitive adhesive, whichcan easily be separated from the adherend with the aid of water and canbe dissolved or dispersed in water without use of any alkali or acid,has been desired. This pressure-sensitive adhesive is preferably anaqueous pressure-sensitive adhesive also from the viewpoint of anenvironmental problem.

Synthetic resin emulsions produced by polymerizing acrylic monomers orthe like in the presence of an emulsifier have hitherto been extensivelyused as the aqueous pressure-sensitive adhesive. By virtue of arelatively large molecular weight of the polymer, such synthetic resinemulsions can exhibit good water resistance in aqueouspressure-sensitive adhesives. Due to this nature, disadvantageously, thedried film is not easily swollen with water. For this reason, in orderto render the polymer in this synthetic resin emulsion swellable withwater, it is common practice to use a large amount of a hydrophilicsurfactant or to add a water-soluble polymer such as polyvinyl alcoholor cellulose ether.

The use of a large amount of the surfactant sometimes causesplasticization of the polymer with the surfactant and consequently,however, disadvantageously causes a change in adhesive properties withthe elapse of time. Further, the use of a large amount of thewater-soluble polymer sometimes enhances the viscosity of thepressure-sensitive adhesive, deteriorates the storage stability due tothe function of the water-soluble polymer as a coagulating agent, ordeteriorates coatability due to structural viscosity.

On the other hand, a method, in which a polymer having a high carboxylgroup content produced by solution polymerization is brought to awater-soluble polymer using an aqueous alkaline solution and emulsionpolymerization is carried out using the water-soluble polymer as anemulsifier, has also been extensively used.

The polymer thus obtained, however, is swellable with water only underalkaline conditions. Further, in the polymerization process, aftersolution polymerization, the polymer is brought to an aqueous alkalisolution followed by emulsion polymerization. This operation istroublesome. Further, this method suffers from an additional problemthat the solvent used in the solution polymerization stays in thesynthetic resin emulsion.

Accordingly, the development of a pressure-sensitive adhesivecomposition, which has excellent basic adhesive properties such asadhesion, cohesive force, and tackiness required of thepressure-sensitive adhesive and, at the same time, can be easily swollenwith water without any alkali treatment, has been desired.

SUMMARY OF THE INVENTION

The present inventors have now found that, when a synthetic resinemulsion containing polymer particles having a core/shell structurecomprising a shell produced by polymerizing an unsaturated carboxylicacid and a hydrophilic comonomer and a core produced by conductingpolymerization in the presence of the shell is used as a main componentof a pressure-sensitive adhesive composition, the resultantpressure-sensitive adhesive composition has excellent fundamentaladhesive properties and can be easily swollen with water. Further, itwas found that, in the preparation of this synthetic resin emulsion, theuse of the copolymer solution for the shell, without neutralization, inthe next polymerization process for the formation of core copolymer wasadvantageous. Further, it was found that monomers, which have beenselected to provide a polymer having a glass transition point (Tg) of−20° C. or below, can be advantageously used as monomers forpolymerization for the formation of the core. The present invention hasbeen made based on such finding.

An object of the present invention is to provide a synthetic resinemulsion that can form a pressure-sensitive adhesive composition whichhas excellent adhesive properties such as adhesive strength, cohesiveforce and tackiness and can be easily swollen with water without anytreatment with an alkali.

According to one aspect of the present invention, there is provided asynthetic resin emulsion comprising polymer particles having acore/shell structure, wherein

said shell comprises a copolymer of an unsaturated carboxylic acid and ahydrophilic comonomer,

said core comprises a copolymer of a monomer mixture comprising aradically polymerizable main monomer and a radically polymerizablefunctional monomer, and said monomers constituting the monomer mixtureare selected so that the glass transition point (Tg) of the copolymerproduced by polymerization is −20° C. or below, and

said synthetic resin emulsion has been produced by adding said monomermixture for core formation and a pH adjustor to an aqueous copolymersolution, which has not been neutralized, produced by polymerizing theunsaturated carboxylic acid and the hydrophilic comonomer in an aqueousmedium, and allowing a polymerization reaction to proceed.

This synthetic resin emulsion is preferably used as a main component ofan easily water-swellable pressure-sensitive adhesive composition.

The easily water-swellable pressure-sensitive adhesive compositionaccording to the present invention comprises, as a main component, theabove synthetic resin emulsion.

According to another aspect of the present invention, there is provideda process for producing a synthetic resin emulsion comprising polymerparticles having a core/shell structure, said shell comprising acopolymer of an unsaturated carboxylic acid and a hydrophilic comonomer,said core comprising a copolymer of a monomer mixture comprising aradically polymerizable main monomer and a radically polymerizablefunctional monomer, said process comprising the steps of:

providing an unsaturated carboxylic acid and a hydrophilic comonomer;polymerizing them in an aqueous medium to prepare an aqueous copolymersolution; and adding said monomer mixture for core formation and a pHadjustor to the aqueous copolymer solution without the neutralization ofthe aqueous copolymer solution, allowing an emulsion polymerizationreaction to proceed to prepare a synthetic resin emulsion, wherein saidmonomer mixture comprising monomers selected so that the glasstransition point (Tg) of the copolymer produced by polymerization is−20° C. or below.

In the synthetic resin emulsion according to the present invention, theshell in the core/shell structure is considered to function also as anemulsifier. Therefore, good polymerization stability can be providedwithout the use of a large amount of other surfactant, protectivecolloid or the like. This can realize the avoidance of use of orreduction of the amount of the surfactant, the protective colloid or thelike which is often causative of adverse effect. Further, thepressure-sensitive adhesive composition using the synthetic resinemulsion according to the present invention is excellent in adhesiveproperties such as adhesive strength, cohesive force, and tackiness, caneasily be swollen with water and dispersed or dissolved in water.Therefore, the pressure-sensitive adhesive composition is advantageousin that adherends can easily be recycled and, further, that, when apressure-sensitive adhesive label or sheet is temporarily used and thenseparated, the pressure-sensitive adhesive composition remainingunremoved on the surface of the adherend can be easily removed.

DETAILED DESCRIPTION OF THE INVENTION

Synthetic Resin Emulsion

The synthetic resin emulsion according to the present invention issuitably used as a main component of an easily water-swellablepressure-sensitive adhesive composition and contains polymer particleshaving a core/shell structure. In the present specification, the term“easily water-swellable” refers to a property that, when water isapplied to the synthetic resin emulsion, the synthetic resin emulsioncan easily be swollen. The expression “polymer particles having acore/shell structure” refers to polymer particles comprising a core partas a center and a shell part which is present so as to cover the corepart. In this case, the shell part is not always required to completelycover the core part and embraces the case where the shell part covers apart of the core part.

The synthetic resin emulsion containing polymer particles having acore/shell structure according to the present invention can be producedby first providing an unsaturated carboxylic acid and a hydrophiliccomonomer, polymerizing them in an aqueous medium to prepare an aqueouscopolymer solution for shell formation, then adding said monomer mixturefor core copolymer formation and a pH adjustor to the aqueous copolymersolution without the neutralization of the aqueous copolymer solution,and allowing a polymerization reaction to proceed to prepare a syntheticresin emulsion. The monomer mixture comprises monomers selected so thatthe glass transition point (Tg) of the copolymer obtained afterpolymerization is −20° C. or below.

Specifically, in the present invention, the copolymer (water-solublepolymer) as the shell is formed in an early stage of the polymerizationand is present as a water-soluble polymer in the polymerization system.The monomer composition, which has been adjusted so that thepolymerization of the monomer composition can provide a polymer having aTg of −20° C. or below, is then added in the presence of the aqueouswater-soluble polymer solution, and a polymerization is allowed toproceed. In this case, in the progress of the polymerization, thewater-soluble polymer for the formation of the shell is considered tofunction also as an emulsifier in the polymerization system. Finally,particles having the so-called “core/shell structure” in which thecopolymer as the core has been covered with the water-soluble polymer asthe shell are formed.

Shell

In the present invention, the shell of the polymer particles having acore/shell structure comprises a copolymer of an unsaturated carboxylicacid with a hydrophilic comonomer. Specifically, the copolymer for theformation of the shell (hereinafter often referred to as “shellpolymer”) is produced by polymerizing an unsaturated carboxylic acid anda hydrophilic comonomer in an aqueous medium in the presence of apolymerization initiator.

Unsaturated Carboxylic Acid

Unsaturated carboxylic acids usable in the present invention include,for example, acrylic acid, methacrylic acid, itaconic acid, crotonicacid, and maleic acid. In the present invention, acrylic acid ormethacrylic acid is preferred, and acrylic acid is particularlypreferred. Acrylic acid is highly reactive with other monomer, canstabilize the polymerization and further is also advantageous from theviewpoint of regulating the balance between water solubility andadhesive properties.

In the present invention, the amount of the unsaturated carboxylic acidused is preferably 1 to 50% by weight, more preferably 2 to 30% byweight, based on the whole monomer including the core/shell. When theamount of the unsaturated carboxylic acid used is in the above-definedrange, advantageously, the swellability with water is good andsensitivity to water is lowered, and, thus, no significant change inadhesive properties during use in everyday life occurs.

Hydrophilic Comonomer

The hydrophilic comonomer usable in the present invention is suitably ahydrophilic comonomer which is other than the above unsaturatedcarboxylic acid and has a solubility in 100 g of water that is not lessthan 2 g. Specific examples of such hydrophilic comonomers includehydroxy esters of (meth)acrylic acid, (meth)acrylic esters having noxyethylene structures on their side chain, vinylpyrrolidone, vinylacetate, N-methylolacrylamide, alkoxymethylacrylamide,dimethylaminoethyl methacrylate, diacetoneacrylamide, N-butylacrylamide,acrylamide, methacrylamide, and phosphate group-containing monomers.

In the present invention, the hydrophilic comonomer is preferably ahydroxy ester of (meth)acrylic acid from the viewpoints ofcopolymerizability with the unsaturated carboxylic acid and easy waterswellability. Among others, hydroxyethyl (meth)acrylate is particularlypreferred.

In the present invention, the amount of the hydrophilic comonomer usedis preferably 1 to 50% by weight, more preferably 3 to 30% by weight,based on the whole monomer including the core/shell. When the amount ofthe hydrophilic comonomer used is in the above-defined range, advantagescan be obtained including that the stability of the polymerization andswellability with water are good, a significant increase in viscosityduring polymerization can be avoided, a stable product can be provided,and stable adhesive properties are provided.

Polymerization Initiator

The polymerization initiator used in the polymerization of anunsaturated carboxylic acid and a hydrophilic comonomer in an aqueousmedium is not particularly limited, so far as the polymerizationinitiator is usable in the polymerization in a conventional aqueoussystem, and a conventional polymerization initiator can be properlyselected.

Such polymerization initiators include, for example, those which areradically decomposed thermally or with a reducing material to allowaddition polymerization of the monomer to proceed, and examples thereofinclude water-soluble or oil-soluble persulfates, peroxides, or azobiscompounds. Specific examples thereof include potassium persulfate,ammonium persulfate, t-butylhydroperoxide, hydrogen peroxide, andazobisisobutyronitrile (AIBN). They may be used either solely or in acombination of two or more of them.

These polymerization initiators may be, if necessary, used incombination with transition metal ions. Preferred transition metal ionsinclude, for example, ferric sulfate, cupric chloride, or ferricchloride.

The polymerization process of the copolymer for the formation of theshell will be instantiated. In the polymerization for the formation ofthe copolymer for the shell, an aqueous medium is first charged into areaction kettle and is then heated, an unsaturated carboxylic acid and ahydrophilic monomer are then added thereto, the mixture is heated, apolymerization initiator is optionally added, and a polymerizationreaction is allowed to proceed. This results in the formation of anaqueous solution (an aqueous polymer solution) in which a transparentwater-soluble polymer is present in a dissolved or dispersed state inwater. Aqueous media usable herein include, for example, water andmixtures of water with alcohols such as ethanol. The aqueous polymersolution is used, without neutralization, in the next step of formingthe polymer as the core.

Core

In the present invention, the core of polymer particles having acore/shell structure is a copolymer which is produced from a monomermixture comprising a radically polymerizable main monomer and aradically polymerizable functional monomer and a glass transition point(Tg) of −20° C. or below. Therefore, monomers constituting the monomermixture are selected so that the copolymer produced by thepolymerization has a Tg of −20° C. or below.

The copolymer for the core (hereinafter often referred to as “corepolymer”) can be formed by forming the copolymer as the shell asdescribed above, adding the above monomer mixture and, further, a pHadjustor to an aqueous solution of this copolymer without neutralizationof the copolymer (that is, in an unneutralized state), and conductingemulsion polymerization.

In the present invention, in such a state that the aqueous solution ofthe copolymer for shell formation (water-soluble polymer) is in anunneutralized state, the polymerization for the formation of a corepolymer in the presence of this copolymer is initiated. The reason whythe aqueous copolymer solution is used, without neutralization, in thesubsequent step of core formation is that, when the step ofneutralization is omitted, pH of the aqueous copolymer (water-solublepolymer) solution can be held in an acidic region, preferably at 7 orless, whereby the polymerization reaction can be allowed to proceedstably. In this case, it is considered that, since only a part of thepolymer component is neutralized during polymerization with a pHadjustor added during the emulsion polymerization, only polymerizationstability (or only storage stability) can be improved withoutsacrificing the reactivity of the polymerization system.

The emulsion polymerization for core copolymer formation is preferablycarried out immediately after the shell polymer formation. In this case,the water-soluble polymer formed by the first polymerization is notpresent solely and is partially copolymerized with the core polymercomponent, and, hence, the water-soluble polymer can be present on asuitable level in the coating after the film formation. As a result,swellability can be further improved.

In the present invention, the monomer used in the formation of the corecopolymer may be any monomer without particular limination so far as themonomer is a radically polymerizable unsaturated monomer used inconventional emulsion polymerization. In the present invention, however,typically, a monomer mixture comprising a radically polymerizable mainmonomer and a radically polymerizale functional monomer is used. Theterm “radically polymerizable main monomer” as used herein is a monomerwhich is a main component of the core polymer and is radicallypolymerizable. The term “radically polymerizable functional monomer” asused herein refers to a functional monomer which can modify the corepolymer to impart further function and is radically polymerizable.

Radically Polymerizable Main Monomer

Radically polymerizable main monomers usable in the present inventioninclude, for example, alkyl esters of (meth)acrylic acid, cycloalkylesters of (meth)acrylic acid, olefins, vinyl esters, and aromatic vinylcompounds.

Specifically, alkyl esters of (meth)acrylic acid or cycloalkyl esters of(meth)acrylic acid as the radically polymerizable main monomer includeesters of alkyls having 1 to 12 carbon atoms such as methyl, ethyl,n-butyl, t-butyl, propyl, 2-ethylhexyl, or octyl with (meth)acrylicacid, cyclohexyl acrylate, and cyclohexyl methacrylate. Olefins includeethylene and propylene. Vinyl esters include vinyl acetate, vinyl estersof branched carboxylic acids, and vinyl laurate. Aromatic vinylcompounds include styrene and α-methylstyrene.

In a preferred embodiment of the present invention, the main monomer isan alkyl ester of (meth)acrylic acid, a cycloalkyl ester of(meth)acrylic acid, styrene, or a vinyl ester of a branched carboxylicacid. They may be used solely. Preferably, however, they are used in acombination of two or more of them.

Specifically, a combination of two or more alkyl esters of (meth)acrylicacid, a combination of two or more alkyl esters of (meth)acrylic acidwith styrene, and a combination of a vinyl ester of a branchedcarboxylic acid with an alkyl ester of methacrylic acid are preferredfrom the viewpoints of weathering resistance, polymerization stabilityand the like.

In a more preferred embodiment of the present invention, a combinationof two or more alkyl esters of (meth)acrylic acid is used as the mainmonomer. Specifically, a combination of 2-ethylhexyl acrylate/butylacrylate/methyl methacrylate or a combination of 2-ethylhexylacrylate/methyl methacrylate is preferred. The reason why the abovecombination is preferred is that the glass transition point (Tg) of thecore copolymer can be regulated to −20° C. or below and various adhesiveproperties can also be advantageously improved.

In the present invention, the amount of the radically polymerizable mainmonomer used is preferably 20 to 97% by weight, more preferably 50 to93% by weight, based on the whole monomer including the core/shell. Whenthe amount of the radically polymerizable main monomer used is more than20% by weight, satisfactory adhesive properties can be ensured as thepressure-sensitive adhesive composition. On the other hand, when theamount of the radically polymerizable main monomer used is less than 97%by weight, satisfactory water swellability can be achieved because it ispossible to prevent the hydrophobicity from being increased to anexcessively high level.

Radically Polymerizable Functional Monomer

Radically polymerizable functional monomers usable in the presentinvention are typically those which can modify synthetic resins toimpart storage stability, water resistance, chemical resistance,weathering resistance, adhesion or other properties to the syntheticresins.

Examples of such functional monomers include: ethylenically unsaturatedcarboxylic acids for improving storage stability and adhesion; theso-called “cross-linkable monomer” having two or more radicallypolymerizable unsaturated bonds for improving water resistance,weathering resistance, chemical resistance, adhesion and the like; andmonomers having on its side chain an alkoxysilyl group, an amide group,a nitrile group, a hydroxyl group, a glycidyl group, a methylol group, acarbonyl group, a quarternary ammonium salt, an ethylene oxide chain, orchlorine.

Specific examples of ethylenically unsaturated carboxylic acids includeacrylic acid, methacrylic acid, crotonic acid, and maleic acid.

Crosslinkable monomers having two or more radically polymerizableunsaturated bonds include, for example, divinyl compounds,di(meth)acrylate compounds, tri(meth)acrylate compounds,tetra(meth)acrylate compounds, diallyl compounds, triallyl compounds,and tetraallyl compounds. More specific examples of crosslinkablemonomers include divinylbenzene, divinyl adipate, ethylene glycoldi(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycoldi(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropyleneglycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,3-butyleneglycol di(meth)acrylate, 1,3-butyl di(meth)acrylate, trimethylolethanetri(meth)acrylate, trimethylolpropane tri(meth)acrylate, dipentaerythrittri(meth)acrylate, diallyl phthalate, triallyl dicyanurate, andtetraallyloxyethane.

Specific examples of other functional monomers include the followingmonomers.

Monomers having an alkoxysilyl group include vinyltriethoxysilane and3-methacryloxypropyltriethoxysilane. Monomers having a hydroxyl groupinclude hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropylmethacrylate, hydroxypropyl acrylate, hydroxybutyl methacrylate, andhydroxybutyl acrylate. Monomers having an amide group include acrylamideand methacrylamide. Monomers having a nitrile group includeacrylonitrile. Monomers having chlorine on its side chain include vinylchloride and vinylidene chloride. Monomers having a glycidyl groupinclude glycidyl methacrylate and glycidyl acrylate. Monomers having amethylol group include N-methylolacrylamide. Monomers having a carbonylgroup include acetoacetoxyethyl methacrylate.

In the present invention, the functional monomer is preferably anethylenically unsaturated carboxylic acid. When the ethylenicallyunsaturated carboxylic acid is used as the functional monomer, by virtueof the presence of the ethylenically unsaturated carboxylic acid, notonly the shell copolymer but also the core polymer can be easily swollenwith water.

In the present invention, the amount of the unsaturated carboxylic acidused as the functional monomer is preferably 1 to 50% by weight, morepreferably 2 to 30% by weight, based on the whole monomer including thecore/shell. When the amount of the unsaturated carboxylic acid used is1% by weight or more, the swellability of the core polymer with watercan be maintained. On the other hand, when the amount of the unsaturatedcarboxylic acid used is 50% by weight or less, the viscosity of theresultant emulsion can be brought to a suitable level and, further,satisfactory adhesive strength can be advantageously provided.

Methods for adding the monomer for forming the core copolymer include abatch polymerization method in which the whole amount of the monomer isadded to the reaction kettle at a time, a dropwise addition method inwhich the monomer is added dropwise, and an emulsion monomer dropwiseaddition method in which the monomer is emulsified with a surfactant (oran emulsifier) and the emulsion is added dropwise.

In the present invention, the dropwise addition method or the emulsionmonomer dropwise addition method is preferably used.

The present invention is characterized in that, in the production of thesynthetic resin emulsion, when the monomer for core copolymer formationis added dropwise, the monomer, together with a pH adjustor, is added.

When the pH adjustor is added together with the monomer for coreformation, since the pH adjustor is added during polymerization for corepolymer formation, partial neutralization can be homogeneously carriedout from within the particles. As a result, for the coating after thefilm formation, the sensitivity to water is higher than that of anunneutralized coating or a post-neutralized coating. As compared withthe case where polymerization is carried out after the neutralization ofthe aqueous copolymer (water-soluble polymer) solution for shellformation, the hydrophilic component is properly incorporated in thepolymer without sacrificing the copolymerizability of the shell polymerand the core polymer. Therefore, the swellability of the coating withwater, i.e., the sensitivity of the coating to water, can be furtherimproved.

pH adjustors usable in the present invention include, for example,alkali metal salts, ammonia, and amine.

In the present invention, the amount of the pH adjustor used ispreferably 1 to 75% by weight, more preferably 3 to 50% by weight, basedon the whole unsaturated carboxylic acid including the core/shell. Theuse of the pH adjustor in an amount of 1% by weight or more is preferredfrom the viewpoints of neutralizing the unsaturated carboxylic acid andensuring swellability with water. When the amount of the pH adjustorused is 75% by weight or less, the viscosity of the emulsion after theneutralization can be brought to a suitable level to improve theworkability of work using the emulsion and, further, the sensitivity tomoisture in the air can be suppressed to stabilize properties duringuse.

Other Components

In the present invention, the polymerization for core copolymerformation may be carried out in an aqueous medium to which the abovemonomer mixture and the pH adjustor are added and conventional othercomponents are further added. For example, a surfactant and/or apolymerization initiator can be used as the conventional othercomponent. Further, for example, protective colloids, chain transferagents, ultraviolet absorbers, and photooxidation inhibitors may also beif necessary used.

In the present invention, the surfactant is one which functions as anauxiliary emulsifier in the emulsion polymerization. Specific examplesof surfactants usable in the present invention include conventionalanionic, cationic or nonionic surfactants.

More specifically, anionic surfactants include, for example, sodiumalkylbenzenesulfonates, sodium alkylsulfonates, and sodiumpolyoxyethylene alkyl ether sulfonates.

Nonionic surfactants include, for example, polyoxyethylene alkyl ether,polyoxyethylene, and polyoxypropylene glycol surfactants.

Further, in the present invention, radically polymerizable surfactantshaving at least one radically polymerizable unsaturated groups in theirmolecule are also usable as the surfactant. The radically polymerizablesurfactant is chemically bonded to the polymer component due to thepresence of the polymerizable unsaturated bond and is known not to bleedout, as a free surfactant, on the surface of the pressure-sensitiveadhesive upon film formation. Therefore, a change in adhesive propertieswith the elapse of time can be inhibited, and, thus, this surfactant canbe favorably used in the present invention.

The radically polymerizable surfactant may be properly selected fromconventional materials, for example, anionic and nonionic surfactants.Specific examples thereof include the following compounds 1) to 15):

 1)

whereinR¹ and R²: H or CH₃;R³: C₇₋₂₁ alkyl or alkenyl group;andM: alkalimetal or ammoniumgroup(see Japanese Patent Laid-OpenPublication No.144317/1979)  2)

whereinR: H or CH₃; andM: alkali metal, ammoniumgroup, or amine(seeJapanese Patent Laid-OpenPublication No. 115419/1980)  3)

whereinR: H or CH₃;A: alkylene group;n: integer of 2 or more; andM:monovalent or divalent cation(see Japanese Patent Laid-OpenPublicationNo. 34947/1987)  4)

whereinR¹: H or CH₃;R²: unsubstituted or substitutedhydrocarbon group orthe like;A: C₂₋₄ alkylene group orsubstituted alkylene group; andn: 0 orpositive number(see Japanese Patent PublicationNo. 46291/1974)  5)

whereinR¹: H or CH₃;R²: unsubstituted or substitutedhydrocarbon group,amino group orthe like;A: C₂₋₄ alkylene group;n: 0 to 100; andM:monovalent or divalent cation(see Japanese Patent Laid-OpenPublicationNo. 203960/1983)  6)

whereinR¹: C₆₋₁₈ alkyl group or the like;R²: H, C₆₋₁₈ alkyl group or thelike;R³: H or propenyl group;A: C₂₋₄ alkylene group orsubstitutedalkylene group;M: alkali metal or the like; andn: 1 to 200(see JapanesePatent Laid-OpenPublication No. 53802/1992)  7)

whereinR¹: H or CH₃;R²: C₈₋₂₄ hydrocarbon group orthe like;A: C₂₋₄alkylene group;M: H, alkali metal, alkaline earthmetal, ammonium groupor the like;L: 0 to 20; andm: 0 to 50(see Japanese PatentLaid-OpenPublication No. 104802/1987)  8)

whereinR: C₈₋₂₂ hydrocarbon group; andM: alkali metal orammoniumgroup(see Japanese Patent Laid-OpenPublication No. 40388/1974) 9)

whereinR: C₈₋₂₂ hydrocarbon group; andM: alkali metal orammoniumgroup(see Japanese Patent Laid-OpenPublication No. 40388/1974)10)

whereinR: alkyl or alkylphenyl;A: ethylene;M: ammonium, amine, oralkalimetal; andm: 9, 12, 14, or 28 (workingexample)(see Japanese PatentLaid-OpenPublication No. 134658/1977) 11)

whereinR¹: H or CH₃;R²: H, CH₃, or —C₆H₄—(CH₂)_(m)—H;andn: 4 to 30(seeJapanese Patent Laid-OpenPublication No. 126093/1978) 12)

whereinR¹ and R²: H or CH₃;x: 0 to 100;y: 0 to 100;z: 0 to 100; and1 ≦x + y + z ≦ 100(see Japanese Patent Laid-OpenPublication No. 28208/1981)13)

whereinR¹: C₆₋₁₈ alkyl group or the like;R²: H, C₆₋₁₈ alkyl group orthelike;R³: H or propenyl group;A: C₂₋₄ alkylene group orsubstitutedalkylene group; andn: 1 to 200(see Japanese Patent Laid-OpenPublicationNo. 50204/1992) 14)

whereinR¹: H or CH₃;R²: C₈₋₂₄ hydrocarbon group oracyl group;A: C₂₋₄alkylene group;L: 0 to 100; andm: 0 to 50(see Japanese PatentLaid-OpenPublication No. 104802/1987) 15)

whereinR¹ and R²: H, C₁₋₂₀ hydrocarbongroup, or acyl group;A¹ and A²:C₂₋₄ alkylene group orsubstituted alkylene group;L: 1 or 2;m and n: 0 orpositive numberwith the proviso that m + n ≧ 3; andwhen both R¹ and R²are H, mand n ≧ 1(see Japanese Patent Laid-OpenPublication No.98484/1975)

In an early stage of the polymerization for core polymer formation, thewhole amount or at least a part of the surfactant can be added dropwiseto the aqueous copolymer solution. Alternatively, a method may beadopted in which the surfactant is previously mixed with the monomermixture used for core polymer formation to bring the monomer mixture toan emulsion monomer solution which is then added to the aqueouscopolymer solution for a polymerization reaction. Further, in theaddition of the surfactant, these embodiments for adding the surfactantmay be used in combination.

In the present invention, since the water-soluble polymer for shellformation functions as an emulsifier, the surfactant is used anauxiliary emulsifier. Therefore, the amount of the surfactant used isgenerally smaller than the amount of the surfactant used in conventionalemulsion polymerization.

The amount of the surfactant used is preferably about 0 (zero) to 2% byweight based on the whole monomer. When the amount of the surfactantused is 2% by weigh or less, for example, contamination of the adherendcaused by bleedout can be prevented.

In the present invention, the polymerization initiator is notparticularly limited so far as it can be radically decomposed thermallyor with a reducing material to allow addition polymerization of themonomer to proceed, and examples thereof include water-soluble oroil-soluble persulfates, peroxides, or azobis compounds. Specificexamples thereof include potassium persulfate, ammonium persulfate,t-butylhydroperoxide, hydrogen peroxide, and azobisisobutyronitrile(AIBN). They may be used either solely or in a combination of two ormore of them. Further, these polymerization initiators may be, ifnecessary, used in combination with transition metal ions. Preferredtransition metal ions include, for example, ferric sulfate, cupricchloride, or ferric chloride.

In the present invention, any protective colloid can be used withoutparticular limitation so far as it is a conventional protective colloidusable in emulsion polymerization. Specific examples thereof includepolyvinyl alcohol and its derivatives, cellulose ether and itsderivatives, and starch derivatives. They are used as an aqueoussolution.

The chain transfer agent is not particularly limited and may be properlyselected from conventional chain transfer agents, and specific examplesthereof include: alcohols, such as methanol, ethanol, propanol, andbutanol; acetone, methyl ethyl ketone, cyclohexane, and acetophenone;carboxylic acids having 2 to 8 carbon atoms, such as acetaldehyde,propionaldehyde, n-butylaldehyde, furfural, and benzaldehyde; andmercaptans, such as dodecyl mercaptan, lauryl mercaptan, normalmercaptan, thioglycolic acid, octyl thioglycolate, and thioglycerol.They may be used either solely or in a combination of two or more ofthem.

The ultraviolet absorber is not particularly limited. However,benzophenone derivatives and benzotriazole derivatives are suitablyused. Some of these ultraviolet absorbers have a radically polymerizableunsaturated bond and are preferred because of copolymerization with thesynthetic resin component.

Preferred photooxidation inhibitors include hindered phenol or hinderedpiperidine photooxidation inhibitors. As with the ultraviolet absorber,some of photooxidation inhibitors have a radically polymerizableunsaturated bond and are preferred because of copolymerization with thesynthetic resin component.

Glass Transition Point

In the present invention, the glass transition point (Tg) of the corecopolymer produced by the polymerization is −20° C. or below, preferably−20° C. to −70° C., more preferably −40° C. to −70° C. When Tg is −20°C. or below, the pressure-sensitive adhesive composition containing thesynthetic resin emulsion has good tackiness.

The term “glass transition point (Tg)” refers to a temperature at whichthe synthetic resin particles contained in the synthetic resin emulsioncauses a phase change from a hard, brittle glass state to a soft,rubbery state. In the determination of the glass transition point, thepresence of an inflection point can be easily confirmed by measurementwith an analyzer such as a differential scanning calorimeter (DSC).Further, regarding the glass transition point (Tg), Tg of the polymercan be easily analogized from the composition of the monomer by thefollowing FOX's equation.

(FOX's Equation)1/Tg=W ₁ /Tg ₁ +W ₂ /Tg ₂ +W ₃ /Tg ₃ + . . . +W _(n) /Tg _(n) W ₁ +W ₂+W ₃ + . . . +W _(n)=1wherein

1 to n represent a positive number;

W₁, W₂, W₃, . . . , and W_(n) respectively represent the weightfractions of the monomers; and

Tg₁, Tg₂, Tg₃, . . . , and Tg_(n) respectively represent the glasstransition points (absolute temperature) of the homopolymers.

Easily Water-swellable Pressure-sensitive Adhesive Composition

The easily water-swellable pressure-sensitive adhesive compositionaccording to the present invention is composed mainly of the syntheticresin emulsion according to the present invention. The expression“composed mainly of” as used herein of course embraces the case wherethe easily water-swellable pressure-sensitive adhesive compositionconsists of the synthetic resin emulsion as the main component only andfurther embraces the case where, in addition to the synthetic resinemulsion as the main component, other optional components may becontained so far as the adhesive properties inherent in the syntheticresin emulsion are not lost. Accordingly, the easily water-swellablepressure-sensitive adhesive composition may contain various conventionalauxiliary components so far as the above synthetic resin emulsion iscontained as the main component.

Such auxiliary components include, for example, tackifying resins,various pigments, dyes, color pigments, thickeners, surfactants,dispersants, antifoaming agents, antifreezing agents, ultravioletabsorbers, and photooxidation inhibitors.

In incorporating these assistants in the pressure-sensitive adhesivecomposition, a method may be adopted in which a formulation containingthese assistants (a pigment paste) is separately provided and is mixedwith and incorporated in the synthetic resin emulsion.

According to the present invention, there is provided use of thesynthetic resin emulsion according to the present invention, as apressure-sensitive adhesive.

The pressure-sensitive adhesive sheet according to the present inventioncomprises a substrate sheet and the above easily water-swellablepressure-sensitive adhesive composition coated onto the surface of thesubstrate sheet.

In the present invention, the easily water-swellable pressure-sensitiveadhesive composition may be coated directly on the substrate sheet toform a coating which is then dried to form a pressure-sensitive adhesivesheet or label. Alternatively, a method may be adopted in which theeasily water-swellable pressure-sensitive adhesive composition is oncecoated onto a separator, the coating is dried, and the coated separatoris then laminated onto a substrate to prepare a pressure-sensitiveadhesive sheet or label.

In the present invention, substrate usable in the pressure-sensitiveadhesive sheet or the like include, for example, wood free paper, coatedpaper, kraft paper, nonwoven fabrics, synthetic paper such as Yupo, andplastic film substrates such as PET, PP, PE, and polyvinyl chloride.

At that time, the coating is carried out by regulating a coater in sucha manner that the coverage is about 15 to 25 g/m². Coaters usable hereininclude conventional coaters such as Komma coater, roll coater, and diecoater.

The pressure-sensitive adhesive sheet or label on which thepressure-sensitive adhesive composition according to the presentinvention has been coated has excellent adhesive properties such asadhesion, tackiness, and cohesive force and, further, can be easilyswollen with even alkali-free water, and does not cause any adhesiveresidue on the adherend and thus has excellent recycling efficiency.

The adherend is not particularly limited, and examples thereof includevarious adherends such as bottles, cans, building materials, packagingmaterials, materials to be packaged, electronic materials, and sanitarygoods.

In another embodiment of the present invention, there is provided abonding method comprising the step of bonding a substrate and a adherendof interest with the aid of the synthetic resin emulsion according tothe present invention.

In still another embodiment of the present invention, there is provideda method for bonding a pressure-sensitive adhesive sheet, comprising thesteps of: coating the above easily water-swellable pressure-sensitiveadhesive composition onto the surface of a substrate sheet to form thepressure-sensitive adhesive sheet; and applying the pressure-sensitiveadhesive sheet to a adherend of interest.

In a further preferred embodiment of the present invention, there isprovided a method for bonding a pressure-sensitive adhesive sheet,comprising the steps of: providing the pressure-sensitive adhesive sheetaccording to the present invention; and applying the pressure-sensitiveadhesive sheet to a adherend of interest.

In another embodiment of the present invention, there is provided amethod for separating a pressure-sensitive adhesive sheet, comprisingthe steps of: applying water to a pressure-sensitive adhesivecomposition-coated part in the above pressure-sensitive adhesive sheetapplied to a adherend of interest to swell the pressure-sensitiveadhesive composition; and then separating the pressure-sensitiveadhesive sheet, in which the pressure-sensitive adhesive composition hasbeen swollen, from the adherend. In this embodiment, the term “water”refers to water per se and further embraces an aqueous medium containinga minor amount of a solvent such as alcohol.

EXAMPLES

The present invention is further illustrated by the following Examplesthat are not intended as a limitation of the invention.

Preparation of Pressure-sensitive Adhesive Compositions

Synthetic resin emulsions were prepared according to the followingprocedure to provide pressure-sensitive adhesive compositions.

Pressure-sensitive Adhesive Composition A1:

Deionized water (100.0 parts by weight), 3.0 parts by weight of acrylicacid, and 9.0 parts by weight of 2-hydroxyethyl acrylate were placed ina flask equipped with a thermometer, a reflux condenser, a droppingfunnel, a nitrogen introduction port, and a stirrer. The contents of theflask were heated to 70° C. with stirring under a nitrogen atmosphere.Thereafter, 0.5 part by weight of ammonium persulfate (a polymerizationinitiator) was added thereto, and a polymerization reaction was allowedto proceed to prepare a water-soluble polymer for shell formation.

Next, a monomer mixture composed of 22.5 parts by weight of butylacrylate, 63.5 parts by weight of 2-ethylhexyl acrylate, and 2.0 partsby weight of methacrylic acid, 0.5 part by weight of ammonium persulfate(a polymerization initiator), and 1.0 part by weight of triethanolaminewere continuously added dropwise with stirring over a period of 4 hr inthe presence of this water-soluble polymer for shell formation. Afterthe completion of the dropwise addition, stirring was continued foradditional 3 hr to allow the polymerization reaction to proceed andconsequently to prepare a polymer as a core. Thus, a pressure-sensitiveadhesive composition (pressure-sensitive adhesive composition A1), thatis a synthetic resin emulsion having a core/shell structure (solidcontent: about 49% by weight), was prepared.

Pressure-sensitive Adhesive Composition A2:

Pressure-sensitive adhesive composition A2 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that, inthe formation of the water-soluble polymer for shell formation by thepolymerization, 0.1 part by weight of a compound containing an allylgroup added to a benzene ring in sodium polyoxyethylene nonyl phenylether sulfonate (ethylene oxide: 20 moles) (polymerizable emulsifier A)was further added as an anionic polymerizable emulsifier.

Pressure-sensitive Adhesive Composition A3:

Pressure-sensitive adhesive composition A3 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that, inthe formation of the polymer for the core by the polymerization in thepresence of the water-soluble polymer for shell formation, the monomermixture was brought to an emulsion monomer using 20 parts by weight ofwater and 0.1 part by weight of anionic polymerizable emulsifier A (acompound containing an allyl group added to a benzene ring in sodiumpolyoxyethylene nonyl phenyl ether sulfonate (ethylene oxide: 20moles)), and the emulsion monomer was added dropwise.

Pressure-sensitive Adhesive Composition A4:

Pressure-sensitive adhesive composition A4 was prepared in the samemanner as in pressure-sensitive adhesive composition A3, except that 5parts by weight of methacrylic acid and 7 parts by weight of acrylamidewere used as monomers for the preparation of the water-soluble polymerfor shell formation instead of acrylic acid and 2-hydroxyethyl acrylateand, further, anionic polymerizable emulsifier B (a compound containingan allyl group added to a benzene ring in sodium polyoxyethylene nonylphenyl ether sulfonate (ethylene oxide: 10 moles)) was used instead ofpolymerizable emulsifier A.

Pressure-sensitive Adhesive Composition A5:

Pressure-sensitive adhesive composition A5 was prepared in the samemanner as in pressure-sensitive adhesive composition A3, except that 9parts by weight of methacrylic acid and 3 parts by weight of2-hydroxyethyl acrylate were used as the monomers for the preparation ofthe water-soluble polymer for shell formation instead of acrylic acidand 9.0 parts by weight of 2-hydroxyethyl acrylate and, further,polymerizable emulsifier C (a compound containing an allyl group addedto a benzene ring in nonionic polyoxyethylene nonyl phenyl ether ofethylene oxide (40 moles)) was used instead of polymerizable emulsifierA.

Pressure-sensitive Adhesive Composition A6:

Pressure-sensitive adhesive composition A6 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that themonomer mixture for the preparation of the polymer for core formationwas changed to a monomer mixture composed of 81 parts by weight of butylacrylate, 2.0 parts by weight of methacrylic acid, and 5 parts by weightof methyl acrylate.

Pressure-sensitive Adhesive Composition A7:

Pressure-sensitive adhesive composition A7 was prepared in the samemanner as in pressure-sensitive adhesive composition A3, except that themonomer mixture for the preparation of the polymer for core formationwas changed to a monomer mixture composed of 33.5 parts by weight ofbutyl acrylate, 30.0 parts by weight of 2-ethylhexyl acrylate, 2.0 partsby weight of methacrylic acid, and 22.5 parts by weight of methylacrylate.

Pressure-sensitive Adhesive Composition A8:

Pressure-sensitive adhesive composition A8 was prepared in the samemanner as in pressure-sensitive adhesive composition A3, except that 5.0parts by weight of acrylic acid and 7.0 parts by weight of hydroxyethylmethacrylate were used as the monomers for the preparation of thewater-soluble polymer for shell formation instead of 3.0 parts by weightof acrylic acid and 9.0 parts by weight of 2-hydroxyethyl acrylate.

Pressure-sensitive Adhesive Composition A9:

Pressure-sensitive adhesive composition A9 was prepared in the samemanner as in pressure-sensitive adhesive composition A3, except that 8.0parts by weight of acrylic acid and 4.0 parts by weight of hydroxyethylmethacrylate were used as the monomers for the preparation of thewater-soluble polymer for shell formation instead of 3.0 parts by weightof acrylic acid and 9.0 parts by weight of 2-hydroxyethyl acrylate and,further, the amount of polymerizable emulsifier A used was changed to1.0 part by weight.

Pressure-sensitive Adhesive Composition A10:

Pressure-sensitive adhesive composition A10 was prepared in the samemanner as in pressure-sensitive adhesive composition A3, except that 9.0parts by weight of hydroxyethyl methacrylate was used as the monomer forthe preparation of the water-soluble polymer for shell formation insteadof 9.0 parts by weight of 2-hydroxyethyl acrylate, the monomer mixturefor the preparation of the polymer for core formation was changed to amonomer mixture composed of 60.0 parts by weight of butyl acrylate, 12.0parts by weight of 2-ethylhexyl acrylate, and 10.0 parts by weight ofmethacrylic acid, and, further, the emulsifier was changed to 1.0 partby weight of nonreactive emulsifier D (polyoxyethylene nonyl phenylether (ethylene oxide: 40 moles)).

Pressure-sensitive Adhesive Composition A11:

Pressure-sensitive adhesive composition A11 was prepared in the samemanner as in pressure-sensitive adhesive composition A9, except that, inthe monomers for the preparation of the water-soluble polymer for shellformation, the amount of hydroxyethyl methacrylate used was changed to20 parts by weight and, further, the amount of triethanolamine used asthe pH adjustor was changed to 1.5 parts by weight.

Pressure-sensitive Adhesive Composition A12:

Pressure-sensitive adhesive composition A12 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that, inthe monomers for the preparation of the water-soluble polymer for shellformation, the amount of hydroxyethyl acrylate used was changed to 20.0parts by weight, the monomer mixture for the preparation of the polymerfor core formation was changed to a monomer mixture composed of 39.0parts by weight of butyl acrylate, 18.0 parts by weight of 2-ethylhexylacrylate, and 20.0 parts by weight of methacrylic acid, and, further,the amount of triethanolamine used as the pH adjustor was changed to 2parts by weight.

Pressure-sensitive Adhesive Composition A13:

Pressure-sensitive adhesive composition A13 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that thepH adjustor used in the formation of the polymer for core formation waschanged from triethanolamine to 4.0 parts by weight of 25% aqueousammonia.

Pressure-sensitive Adhesive Composition A14:

Pressure-sensitive adhesive composition A14 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that thepH adjustor used in the formation of the polymer for core formation waschanged from triethanolamine to 1.5 parts by weight of sodium hydroxide.

Pressure-sensitive Adhesive Composition B1 (Comparative Example): wherePolymerization was Carried out without Formation of Polymer for ShellFormation

Deionized water (80.0 parts by weight) and 0.1 part by weight of acompound containing an allyl group added to a benzene ring in sodiumpolyoxyethylene nonyl phenyl ether sulfonate (ethylene oxide: 20 moles)as an anionic polymerizable emulsifier (polymerizable emulsifier A) wereplaced in a flask equipped with a thermometer, a reflux condenser, adropping funnel, a nitrogen introduction port, and a stirrer. Thecontents of the flask were heated to 70° C. with stirring under anitrogen atmosphere. Thereafter, a monomer mixture composed of 22.5parts by weight of butyl acrylate, 63.5 parts by weight of 2-ethylhexylacrylate, and 2.0 parts by weight of methacrylic acid was brought to anemulsion monomer using 20.0 parts by weight of water and 0.1 part byweight of polymerizable emulsifier A. Next, this emulsion monomer,together with 0.5 part by weight of ammonium persulfate (apolymerization initiator), and 1.0 part by weight of triethanolamine,were continuously added dropwise over a period of 4 hr to the flask.After the completion of the dropwise addition, stirring was continuedfor additional 3 hr to allow the polymerization reaction to proceed andconsequently to prepare a synthetic resin emulsion pressure-sensitiveadhesive composition having a solid content of about 46% by weight(pressure-sensitive adhesive composition B1).

Pressure-sensitive Adhesive Composition B2 (Comparative Example): whereUnsaturated Carboxylic Acid only was used in Formation of Polymer forShell Formation

Pressure-sensitive adhesive composition B2 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that, inthe formation of the water-soluble polymer for shell formation, 100.0parts by weight of deionized water and 12.0 parts by weight of acrylicacid were placed in the flask, the contents of the flask were heated to70° C. with stirring under a nitrogen atmosphere, 0.5 part by weight ofammonium persulfate (a polymerization initiator) was then added thereto,and a polymerization reaction was allowed to proceed.

Pressure-sensitive Adhesive Composition B3 (Comparative Example): whereHydroxyl-containing Monomer only was used in Formation of Polymer forShell Formation

Pressure-sensitive adhesive composition B3 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that, inthe formation of the water-soluble polymer for shell formation, 100.0parts by weight of deionized water and 12.0 parts by weight of2-hydroxyethyl methacrylate were placed in the flask, the contents ofthe flask were heated to 70° C. with stirring under a nitrogenatmosphere, 0.5 part by weight of ammonium persulfate (a polymerizationinitiator) was then added thereto, and a polymerization reaction wasallowed to proceed.

Pressure-sensitive Adhesive Composition B4 (Comparative Example): whereNeutralization was Carried out after Polymerization for Core Formation

Pressure-sensitive adhesive composition B4 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that, inthe monomers for the preparation of the water-soluble polymer for shellformation, 9 parts by weight of 2-hydroxyethyl methacrylate was usedinstead of 2-hydroxyethyl acrylate, and, in the formation of the corepolymer, the polymerization reaction was allowed to proceed withoutdropwise addition of triethanolamine, and, after the completion of thepolymerization reaction, pH was adjusted to 4 by the addition of 1.0part by weight of triethanolamine.

Pressure-sensitive Adhesive Composition B5 (Comparative Example): whereNeutralization was Carried out in Formation of Polymer for ShellFormation

Pressure-sensitive adhesive composition B5 was prepared in the samemanner as in pressure-sensitive adhesive composition A1, except that, inthe formation of the water-soluble polymer for shell formation, 100.0parts by weight of deionized water, 3.0 parts by weight of acrylic acid,and 9.0 parts by weight of 2-hydroxyethyl methacrylate were placed inthe flask, the contents of the flask were heated to 70° C. with stirringunder a nitrogen atmosphere, 0.5 part by weight of ammonium, persulfate(a polymerization initiator) and 1.0 part by weight of triethanolaminewere then added thereto, and a polymerization reaction was allowed toproceed.

Pressure-sensitive Adhesive Composition B6 (Comparative Example):

Pressure-sensitive adhesive composition B5 was prepared in the samemanner as in pressure-sensitive adhesive composition B1, except that themonomer mixture for the preparation of the polymer for core formationwas changed to a monomer mixture composed of 12.0 parts by weight of2-hydroxyethyl methacrylate, 22.5 parts by weight of butyl acrylate,63.5 parts by weight of 2-ethylhexyl acrylate, and 2.0 parts by weightof methacrylic acid.

The formulations of the above pressure-sensitive adhesive compositionswere summarized in Tables 1 and 2. In these tables, all values are inparts by weight.

TABLE 1 Pressure-sensitive adhesive composition A1 A2 A3 A4 A5 A6 A7 A8A9 A10 A11 A12 A13 A14 Shell Water 100 100 80 80 80 100 80 80 80 80 80100 100 100 AA 3 3 3 3 3 5 8 3 8 3 3 3 MAA 5 9 2-HEMA 7 4 9 20 2-HEA 9 99 3 9 9 20 9 9 AM 7 R-SA (A) 0.1 Subtotal 112.0 112.1 92.0 92.0 92.0112.0 92.0 92.0 92.0 92.0 108.0 123.0 112.0 112.0 Core Water 0 0 20 2020 0 20 20 20 20 20 0 0 0 BA 22.5 22.5 22.5 22.5 22.5 81 33.5 22.5 22.560 22.5 39 22.5 22.5 2-EHA 63.5 63.5 63.5 63.5 63.5 30 63.5 63.5 12 63.518 63.5 63.5 MAA 2 2 2 2 2 2 2 2 2 10 2 20 2 2 MA 5 22.5 R-SA (A) 0.10.1 0.1 1 1 R-SA (B) 0.1 R-SA (C) 0.1 N-SA (D) 1 25% Aqueous 4 ammoniaTriethanolamine 1 1 1 1 1 1 1 1 1 1 1.5 2 Sodium 1.5 hydroxide Subtotal89.0 89.0 89.1 89.1 89.1 89.0 89.1 89.1 90.0 84.0 90.5 79.0 92.0 89.5Total 201.0 201.1 181.1 181.1 181.1 201.0 181.1 181.1 182.0 176.0 198.5202.0 204.0 201.5

TABLE 2 Pressure-sensitive adhesive composition B1 B2 B3 B4 B5 B6 ShellWater 80 100 100 100 100 80 AA 12 3 3 MA 2-HEMA 12 9 9 2-HEA R-SA (A)0.1 0.1 Triethanolamine 1 Subtotal 80.1 112 112 112 113 80.1 Core Water20 0 0 0 0 20 MAA BA 22.5 22.5 22.5 22.5 22.5 22.5 2-EHA 63.5 63.5 63.563.5 63.5 63.5 MAA 2 2 2 2 2 2 2-HEMA 12 R-SA (A) 0.1 0.1Triethanolamine 1 1 1 1 1 Sodium hydroxide Subtotal 109.1 89 89 89 88121.1 Total 189.2 201 201 201 201 201.2

Measurement of Glass Transition Point (Tg) and pH

The glass transition point (Tg) of the core polymer part of each of thepressure-sensitive adhesive compositions thus obtained was calculated bythe FOX's equation. Further, pH was also measured for thepressure-sensitive adhesive compositions.

Preparation of Pressure-sensitive Adhesive Sheets

Each of the pressure-sensitive adhesive emulsions prepared above wascoated onto a 25 μm-thick PET film having a surface subjected to coronatreatment (tradename: Lumirror #25, available from Toray Industries,Inc.), and the coating was dried at 100° C. for one min. Thus, apressure-sensitive adhesive sheet was prepared for each of thepressure-sensitive adhesive compositions. In all the cases, the coverageof the pressure-sensitive adhesive composition was about 20 g/m² on adry basis.

Evaluation Tests

The following adhesive properties were examined and evaluated for thepressure-sensitive adhesive sheets which had been prepared usingpressure-sensitive adhesive compositions A1 to A12 andpressure-sensitive adhesive compositions B1 to B6. Base on the obtaineddata on these adhesive properties, preferred property balance forapplications such as pressure-sensitive adhesive sheets was evaluatedoverall as adhesive property balance. The swellability of thepressure-sensitive adhesive compositions was also evaluated.

Evaluation Test a: Adhesive Strength

A pressure-sensitive adhesive sheet cut into a size of width 25 mm andlength 100 mm was applied to a 2.0 mm-thick stainless steel plate (SUS304) which had been polished with sand paper #400 (manufactured byNAGATSUKA ABRASIVE MFG.□CO.,□LTD.), and, in this state, the assembly wasallowed to stand for 24 hr. Thereafter, 180-degree peel strength wasmeasured at a peel rate of 300 mm/min in an environment of temperature23° C. and humidity 65% RH. The peel strength was measured according tothe method specified in JIS Z 0237 and was expressed in N/25 mm.

Evaluation Test b: Cohesive□Force

A pressure-sensitive adhesive sheet cut into a size of width 25 mm andlength 100 mm was applied to a 2.0 mm-thick stainless steel plate (SUS304) which had been polished with sand paper #400 so that theapplication area was 25 mm×25 mm. For the pressure-sensitive adhesivesheet and the stainless steel sheet, a load of 1 kg was applied in anenvironment of 40° C. to measure the time (min) necessary for thepressure-sensitive adhesive sheet to fall from the stainless steelplate.

Evaluation Test c: Tackiness

Tackiness (ball No.) was measured by the J. Dow rolling ball method (JISZ 0237) with an inclined-type ball tack measuring apparatus in anenvironment of temperature 23° C. and RH 65%.

Evaluation of Adhesive Property Balance

The adhesive property balance was judged based on the results ofmeasurement of evaluations a to c. The judgment was done based on thefollowing criteria. Specifically, for the judgment of the adhesiveproperty balance, the results obtained for the evaluation items, i.e.,adhesive strength, cohesive force, and tackiness, were evaluatedaccording to three grades, i.e., very good, good, and failure. When allthe evaluation items were judged to be “very good,” the adhesiveproperty balance was regarded as very good (that is, “AA”). When evenany one of the evaluation items was judged to be “good,” the adhesiveproperty balance was regarded as good (that is, “A”). Further, when evenany one of the evaluation items was judged to be “failure,” the adhesiveproperty balance was regarded as failure (that is, “C”). Specifically,the adhesive property balance was judged as follows.

AA: Very good

A: Good

B: Usable in pressure-sensitive adhesive sheets

C: Failure

Evaluation Test d: Water Swellability

Each of the pressure-sensitive adhesive compositions was coated onto aglass plate (slide glass, manufactured by MATSUNAMI GLASS KOGYO KK) withan applicator (manufactured by YOSHIMITSU SEIKI COMPANY LIMITED) at acoverage of 20 g/m², and the coated glass plates were allowed to standfor 24 hr. Thereafter, the coated glass plates were dipped in tap waterof 23° C. for one hr, and the state of swelling of eachpressure-sensitive adhesive composition coating formed on the glassplate was then observed. The results were judged based on the followingcriteria.

A: The coating was swollen and could be easily removed.

B: The coating was partially swollen.

C: The pressure-sensitive adhesive composition coating was not swollenand could not be easily removed.

The results were as shown in Tables 3 and 4. In Tables 3 and 4,abbreviations have the following means.

AA: Acrylic acid

MAA: Methacrylic acid

2-HEMA: Hydroxyethyl methacrylate

2-HEA: 2-Hydroxyethyl acrylate

AM: Acrylamide

BA: Butyl acrylate

2-EHA: 2-Ethylhexyl acrylate

MA: Methyl acrylate

R-SA (A): Polymerizable emulsifier (reactive emulsifier) A

R-SA (B): Polymerizable emulsifier (reactive emulsifier) B

R-SA (C): Polymerizable emulsifier (reactive emulsifier) C

N-SA (D): Nonreactive emulsifier D

TABLE 3 Pressure-sensitive adhesive composition A1 A2 A3 A4 A5 A6 A7 A8A9 A10 A11 A12 A13 A14 Tg of core polymer −64 −64 −64 −64 −64 −47 −44−64 −64 −40 −64 −24 −64 −64 pH 3.9 3.9 3.9 3.7 3.2 3.9 3.9 3.3 2.9 3.33.5 2.8 3.9 3.9 Adhesive Adhesive 8.1 8.3 8.2 7.7 7.5 7.7 8.8 8.2 8.07.3 8.0 6.9 7.9 8.1 properties strength (N/25 mm) Cohesive 88 76 82 120220 118 95 121 98 77 98 132 90 77 force (min) Tackiness 2 2 2 1 2 2 2 22 2 2 1 2 2 (ball No.) Balance A A A AA AA AA A AA A A A AA A ASwellability A A A A A A A A A A A A A A

TABLE 4 Pressure-sensitive adhesive composition B1 B2 B3 B4 B5 B6 Tg ofcore polymer −64 −64 −64 −64 −64 −53 pH 6.8 2.7 3.1 4.2 4.4 6.6 AdhesiveAdhesive 11.4 5.6 7.1 8.1 8.6 4.8 properties strength (N/25 mm) Cohesive880 230 110 30 11 33 force (min) Tackiness 9 <1 3 1 1 <1 (ball No.)Balance AA C A C C C Swellability C C C C A C

1. A process for producing a synthetic resin emulsion comprising polymerparticles having a core and shell structure, said shell including acopolymer of an unsaturated carboxylic acid and a hydrophilic comonomer,said core comprising a copolymer of a core monomer mixture including aradically polymerizable main monomer and a radically polymerizablefunctional monomer, said process comprising the steps of: providing anunsaturated carboxylic acid and a hydrophilic comonomer; polymerizingsaid unsaturated carboxylic acid and a hydrophilic comonomer in anaqueous medium to form an aqueous copolymer solution; and adding saidcore monomer mixture for core formation when the aqueous copolymersolution is in an unneutralized state to form a reaction mixture;concurrently emulsion polymerizing the reaction mixture and adding a pHadjuster to the aqueous copolymer solution, to form a synthetic resinemulsion, wherein said monomer mixture comprises monomers selected sothat the glass transition point (Tg) of the core copolymer produced bypolymerization is −20° C. or below.
 2. The process for producing asynthetic resin emulsion according to claim 1, wherein, said coremonomer mixture is added as an emulsion monomer solution that isprepared with an emulsifier.
 3. A method for making a pressure-sensitiveadhesive comprising the step of adding a synthetic resin emulsion thatis prepared according to claim 1 to the pressure sensitive adhesiveduring formation of the pressure sensitive adhesive.
 4. A process forproducing a synthetic resin emulsion suitable for use in a pressuresensitive adhesive, wherein said emulsion has polymer particles with acore/shell structure, said process comprising the steps of: a)polymerizing an unsaturated carboxylic acid monomer and a hydrophiliccomonomer in an aqueous medium to produce a shell copolymer for theshell structure of the emulsion particles, where the copolymer is insolution in the aqueous medium; b) combining core monomers with theaqueous medium to form a reaction mixture, and emulsion polymerizing thereaction mixture while concurrently at least partially neutralizing theshell copolymer in the aqueous medium by addition of a pH adjustingagent, to provide a synthetic resin emulsion having a core/shellstructure which exhibits swelling in water without any alkali treatment,wherein said core monomers comprise radically polymerizable main monomerand a radically polymerizable functional monomer in the aqueous medium,and wherein said core monomers are selected such that the core polymerhas a Tg of less than −20° C.
 5. The process according to claim 4,wherein shell copolymer is in an unneutralized state when the coremonomers and pH adjusting agent are added to the aqueous medium.
 6. Theprocess according to claim 4, wherein the core monomers are polymerizedin the reaction mixture, while the pH of the reaction mixture is held atless than 7.